1. Field of the Invention
This invention relates to a portable electronic device which requires auxiliary battery backup and more particularly to a circuit for controlling charge (and discharge) of the auxiliary battery.
2. Description of the Relevant Art
A portable electronic device can be powered from several sources. When available, electronic devices are powered from an AC power supply. Portable electronic devices, however, generally utilize power management architecture and can be powered from a self-contained source such as a battery. Portable electronic devices such as laptop computers, employ sophisticated power management for reducing power consumption during times in which the portable device is not fully utilized. Thus, power management implies standby, suspend and off modes of operation depending upon how much of the portable device is being used. Power management is generally described in U.S. Pat. No. 5,167,024 to Smith, et al. (incorporated herein by reference). Power management of a low power portable device has made prevalent use of a battery as the device's main power source when the AC power supply is unavailable. The battery can maintain delivery of power for several hours depending upon the device's mode of operation. If fully operational, the device will discharge the battery faster than it would if in suspend, standby or off states.
The battery not only serves as the main power source when AC power is unavailable, but also must maintain the memory states of any dynamic RAM memory within the portable device. If, for example, the portable device employs dynamic RAM (DRAM), then the DRAM must be periodically refreshed. If not refreshed, the stored states within DRAM will be lost. It is therefore crucial that the battery maintain enough power to periodically refresh DRAM.
A portable device is oftentimes operated for several hours from a battery source. During operation, the battery source can discharge to significantly low levels. After prolonged operation, the battery may discharge well below a minimum level necessary to maintain DRAM refresh rate. It is therefore important that a minimum charge be maintained to ensure retainage of DRAM state.
Many modern portable electronic devices use an auxiliary battery in addition to a main battery. The main battery provides a main source of power for operating the portable device when AC power is unavailable. The main battery is therefore much larger than and provides greater amounts of power than the auxiliary battery. The auxiliary battery is therefore primarily used for maintaining DRAM state and not for powering the portable device. Unfortunately, however, when the main battery becomes unduly discharged, is disconnected or fails, the auxiliary battery will attempt to power the portable device. The auxiliary battery, however, will not be able to maintain operation power for long periods of time and, if called upon to do so, the auxiliary battery will discharge below a minimum level necessary to refresh DRAM.
FIG. 1 illustrates an exemplary configuration for powering a DRAM-based portable electronic device 10. Electronic device 10 receives power from an AC power source 12, when available. When absent an AC power source 12, electronic device 10 receives power from a main battery 14. Main battery 14 receives trickle charge from an available AC power supply 12 through resistor 16 placed in parallel with diode 18. When AC power supply 12 is removed, main battery 14 provides operational power through diode 18 to device 10. Main battery 14 (or AC power source 12) provide trickle charge through resistor 20 to auxiliary battery 22.
As described above, auxiliary battery 22 tries to charge main battery 14 when main battery 14 is overdischarged, or when auxiliary battery 22 charge is greater than main battery 14 charge. As defined herein, "overdischarge" refers to a discharge less than 0.8 volts per cell when employing NiMH or NiCAD battery cells. Auxiliary battery 22 will become overdischarged if it tries to charge an overly discharged main battery 14 through diode 24. Overdischarge of main battery 14 can be caused by several reasons, including one or more defective cells within main battery 14, or by failure in detecting an overdischarged condition of main battery 14.
Prior art auxiliary battery charge control circuit 26 (shown encompassed in dashed lines) suffers numerous disadvantages, some of which are described above. While circuit 26 provides trickle charge to auxiliary battery 22 during times in which AC power source 12 is present, or when main battery is adequately charged, it does not provide for cessation of discharge of auxiliary battery when AC source is absent and/or when main battery is overdischarged. Thus, circuit 26 is not designed to prevent overdischarge of auxiliary battery 22 below safe levels necessary to maintain DRAM status.